U.S. patent number 9,679,684 [Application Number 14/662,625] was granted by the patent office on 2017-06-13 for voltage nonlinear resistive element and method for manufacturing the same.
This patent grant is currently assigned to NGK Insulators, Ltd.. The grantee listed for this patent is NGK Insulators, Ltd.. Invention is credited to Toru Hayase, Shinji Kawasaki, Kenji Morimoto, Yukihisa Takeuchi, Satoshi Yamazaki.
United States Patent |
9,679,684 |
Yamazaki , et al. |
June 13, 2017 |
Voltage nonlinear resistive element and method for manufacturing
the same
Abstract
A voltage nonlinear resistive element includes a resistor
containing a joined body in which a zinc oxide ceramic layer
composed mainly of zinc oxide and having a volume resistivity of
1.0.times.10.sup.-1 .OMEGA.cm or less is joined to a bismuth oxide
layer composed mainly of bismuth oxide, and a pair of electrodes
disposed on the resistor such that an electrically conductive path
passes through a joint surface between the zinc oxide ceramic layer
and the bismuth oxide layer. In this element, the zinc oxide
ceramic layer of the joined body has a lower volume resistivity
than before. This can result in a lower clamping voltage in a
high-current region than before.
Inventors: |
Yamazaki; Satoshi (Nagoya,
JP), Hayase; Toru (Nagoya, JP), Morimoto;
Kenji (Kasugai, JP), Kawasaki; Shinji (Nagoya,
JP), Takeuchi; Yukihisa (Nagoya, JP) |
Applicant: |
Name |
City |
State |
Country |
Type |
NGK Insulators, Ltd. |
Nagoya |
N/A |
JP |
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Assignee: |
NGK Insulators, Ltd. (Nagoya,
JP)
|
Family
ID: |
52684148 |
Appl.
No.: |
14/662,625 |
Filed: |
March 19, 2015 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20150270036 A1 |
Sep 24, 2015 |
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Foreign Application Priority Data
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Mar 19, 2014 [JP] |
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2014-056497 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01C
7/12 (20130101); C04B 35/62695 (20130101); H01C
17/00 (20130101); H01C 7/18 (20130101); C04B
37/005 (20130101); H01C 7/102 (20130101); C04B
35/453 (20130101); H01C 7/112 (20130101); C04B
37/006 (20130101); H01C 7/10 (20130101); C04B
2235/3277 (20130101); C04B 2237/34 (20130101); C04B
2235/6586 (20130101); C04B 2235/3217 (20130101); C04B
2235/443 (20130101); C04B 2235/3286 (20130101); C04B
2235/5454 (20130101); C04B 2235/96 (20130101); C04B
2235/661 (20130101); H01C 7/1006 (20130101); C04B
2235/322 (20130101); C04B 2237/126 (20130101); C04B
2237/72 (20130101); Y10T 29/49082 (20150115); C04B
2235/5436 (20130101); C04B 2237/06 (20130101); C04B
2235/3298 (20130101); C04B 2235/658 (20130101); C04B
2237/708 (20130101); C04B 2235/3284 (20130101); C04B
2237/125 (20130101); C04B 2235/3267 (20130101); C04B
2235/3294 (20130101) |
Current International
Class: |
H01C
7/112 (20060101); H01C 7/18 (20060101); H01C
17/00 (20060101); C04B 35/453 (20060101); C04B
37/00 (20060101); C04B 35/626 (20060101); H01C
7/12 (20060101); H01C 7/102 (20060101); H01C
7/10 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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764 693 |
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Jan 1957 |
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GB |
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2000-228302 |
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Aug 2000 |
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JP |
|
Other References
Partial European Search Report (Application No. 15159814.1) dated
Sep. 17, 2015. cited by applicant .
Extended European Search Report (Application No. 15159814.1) dated
Jan. 8, 2016. cited by applicant .
U.S. Appl. No. 14/662,669, filed Mar. 19, 2015, Yamazaki, et al.
cited by applicant.
|
Primary Examiner: Harvey; James
Attorney, Agent or Firm: Burr & Brown, PLLC
Claims
What is claimed is:
1. A voltage nonlinear resistive element, comprising: a voltage
nonlinear resistor containing at least two joined bodies in each of
which a zinc oxide ceramic layer composed mainly of zinc oxide and
having a volume resistivity of 1.0.times.10.sup.-1 .OMEGA.cm or
less is joined to a bismuth oxide layer composed mainly of bismuth
oxide, and a pair of electrodes disposed on the voltage nonlinear
resistor such that an electrically conductive path passes through a
junction between the zinc oxide ceramic layer and the bismuth oxide
layer, wherein (i) a bismuth oxide layer, or (ii) a bismuth oxide
layer and a conductor layer, or (iii) a bismuth oxide layer, a
conductor layer, and a bismuth oxide layer are disposed between
adjacent zinc oxide ceramic layers.
2. The voltage nonlinear resistive element according to claim 1,
wherein the zinc oxide ceramic layer contains one or more selected
from the group consisting of Al.sub.2O.sub.3, In.sub.2O.sub.3, and
Ga.sub.2O.sub.3.
3. The voltage nonlinear resistive element according to claim 1,
wherein the bismuth oxide layer is formed on the zinc oxide ceramic
layer by sputtering.
4. A method for manufacturing a voltage nonlinear resistive
element, comprising the steps of: (a) producing a joined body by
forming a bismuth oxide layer composed mainly of bismuth oxide on a
zinc oxide ceramic layer composed mainly of zinc oxide and having a
volume resistivity of 1.0.times.10.sup.-1 .OMEGA.cm or less by
sputtering, (b) preparing at least two of the joined bodies,
stacking the joined bodies with a conductor foil interposed between
the bismuth oxide layer of one of the joined bodies and the bismuth
oxide layer of an adjacent joined body, or directly stacking the
joined bodies without anything interposed therebetween, and
heat-treating the joined bodies in stacked state in an inert
atmosphere at a temperature in the range of 300.degree. C. to
700.degree. C., thereby producing a layered voltage nonlinear
resistor in which the joined bodies are stacked and bonded to each
other, and (c) forming a pair of electrodes such that an
electrically conductive path passes through a junction between the
zinc oxide ceramic layer and the bismuth oxide layer.
5. A method for manufacturing a voltage nonlinear resistive
element, comprising the steps of: (a) producing a joined body by
forming a bismuth oxide layer composed mainly of bismuth oxide on a
zinc oxide ceramic layer composed mainly of zinc oxide and having a
volume resistivity of 1.0.times.10.sup.-1 .OMEGA.cm or less by
sputtering, (b) preparing at least two of the joined bodies,
stacking the joined bodies with a conductor foil interposed between
the bismuth oxide layer of one of the joined bodies and the zinc
oxide ceramic layer of an adjacent joined body, or directly
stacking the joined bodies without anything interposed
therebetween, and heat-treating the joined bodies in stacked state
in an inert atmosphere at a temperature in the range of 300.degree.
C. to 700.degree. C., thereby producing a voltage nonlinear
resistor in which the joined bodies are stacked and bonded to each
other, and (c) forming a pair of electrodes such that an
electrically conductive path passes through a junction between the
zinc oxide ceramic layer and the bismuth oxide layer.
6. The method for manufacturing a voltage nonlinear resistive
element according to claim 4, wherein the heat treatment
temperature in the step (b) ranges from 300.degree. C. to
500.degree. C.
7. The method for manufacturing a voltage nonlinear resistive
element according to claim 5, wherein the heat treatment
temperature in the step (b) ranges from 300.degree. C. to
500.degree. C.
Description
BACKGROUND OF INVENTION
1. Field of Invention
The present invention relates to a voltage nonlinear resistive
element and a method for manufacturing the voltage nonlinear
resistive element.
2. Description of Related Art
Voltage nonlinear resistive elements (varistors) are elements
having a structure in which a voltage nonlinear resistor is
disposed between a pair of electrodes, and are widely used as
elements for protecting electronic circuits from abnormal voltage
in various sensors, such as humidity sensors and temperature
sensors. As a voltage nonlinear resistive element of this type,
Patent Literature 1 discloses a voltage nonlinear resistive element
that includes a resistor between a pair of electrodes, wherein the
resistor has a structure in which an oxide film containing two
oxides, such as Bi.sub.2O.sub.3 and Sb.sub.2O.sub.3, is disposed
between two zinc oxide porcelains containing an oxide, such as
Al.sub.2O.sub.3, as a dopant. This element can decrease the turn-on
voltage V.sub.1mA (a terminal voltage at an electric current of 1
mA (0.4 A/cm.sup.2 based on the shape of the element)) to
approximately 3 V.
CITATION LIST
PTL 1: JP 2000-228302 A
SUMMARY OF THE INVENTION
In general, zinc oxides containing a dopant have lower resistance
than zinc oxides free of dopants. Although the zinc oxide
porcelains described in Patent Literature 1 are doped with 0.0009%
to 0.018% by mass Al.sub.2O.sub.3, the resistance of the zinc oxide
porcelains is not sufficiently low due to the very low dopant
level. It is therefore highly likely that the voltage nonlinear
resistive element described in Patent Literature 1 produces a high
voltage in a high-current region (for example, in the case that an
electric current of 20 A/cm.sup.2 flows).
The present invention solves such problems and principally aims to
lower the clamping voltage of a zinc oxide voltage nonlinear
resistive element in a high-current region.
The present invention is directed to a voltage nonlinear resistive
element including:
a voltage nonlinear resistor containing at least one joined body in
which a zinc oxide ceramic layer composed mainly of zinc oxide and
having a volume resistivity of 1.0.times.10.sup.-1 .OMEGA.cm or
less is joined to a bismuth oxide layer composed mainly of bismuth
oxide, and
a pair of electrodes disposed on the voltage nonlinear resistor
such that an electrically conductive path passes through a junction
between the zinc oxide ceramic layer and the bismuth oxide
layer.
In this voltage nonlinear resistive element, the zinc oxide ceramic
layer of the voltage nonlinear resistor is a zinc oxide ceramic
layer having a lower volume resistivity than before. Thus, the
clamping voltage in a high-current region (for example, in the case
that an electric current of 20 A/cm.sup.2 flows) can be lower than
before. As a result, for example, even when a large current flows
through a voltage nonlinear resistive element according to the
present invention due to static electricity, a rise in voltage can
be decreased, and the dielectric breakdown of the element can be
prevented.
In a voltage nonlinear resistive element according to the present
invention, the zinc oxide ceramic layer may contain one or more
selected from the group consisting of Al.sub.2O.sub.3,
In.sub.2O.sub.3, and Ga.sub.2O.sub.3. The addition of such a
trivalent metal ion can relatively easily decrease the volume
resistivity of the zinc oxide ceramic layer.
In a voltage nonlinear resistive element according to the present
invention, the bismuth oxide layer may be formed on the zinc oxide
ceramic layer by sputtering. This allows the bismuth oxide layer to
be formed without exposing the zinc oxide ceramic layer to a high
temperature. Thus, the volume resistivity of the zinc oxide ceramic
layer is not increased by heat.
In a voltage nonlinear resistive element according to the present
invention, the voltage nonlinear resistor may include two or more
of the joined bodies stacked, and a bismuth oxide layer, or a
bismuth oxide layer and a conductor layer, or a bismuth oxide
layer, a conductor layer, and a bismuth oxide layer may be disposed
between adjacent zinc oxide ceramic layers. Thus, the number of
stacked joined bodies in the voltage nonlinear resistor can be
adjusted for various varistor voltages.
The present invention is also directed to a method for
manufacturing a voltage nonlinear resistive element, comprising the
steps of:
(a) producing a joined body by forming a bismuth oxide layer
composed mainly of bismuth oxide on a zinc oxide ceramic layer
composed mainly of zinc oxide and having a volume resistivity of
1.0.times.10.sup.-1 .OMEGA.cm or less by sputtering,
(b) preparing at least two of the joined bodies, stacking the
joined bodies with a conductor foil interposed between the bismuth
oxide layer of one of the joined bodies and the zinc oxide ceramic
layer of an adjacent joined body, or directly stacking the joined
bodies without anything interposed therebetween, and heat-treating
the joined bodies in stacked state in an inert atmosphere at a
temperature in the range of 300.degree. C. to 700.degree. C.,
thereby producing a voltage nonlinear resistor in which the joined
bodies are stacked and bonded to each other, and
(c) forming a pair of electrodes such that an electrically
conductive path passes through a junction between the zinc oxide
ceramic layer and the bismuth oxide layer,
or alternatively a method for manufacturing a voltage nonlinear
resistive element, comprising the steps of:
(a) producing a joined body by forming a bismuth oxide layer
composed mainly of bismuth oxide on a zinc oxide ceramic layer
composed mainly of zinc oxide and having a volume resistivity of
1.0.times.10.sup.-1 .OMEGA.cm or less by sputtering,
(b) preparing at least two of the joined bodies, stacking the
joined bodies with a conductor foil interposed between the bismuth
oxide layer of one of the joined bodies and the bismuth oxide layer
of an adjacent joined body, or directly stacking the joined bodies
without anything interposed therebetween, and heat-treating the
joined bodies in stacked state in an inert atmosphere at a
temperature in the range of 300.degree. C. to 700.degree. C.,
thereby producing a layered voltage nonlinear resistor in which the
joined bodies are stacked and bonded to each other, and
(c) forming a pair of electrodes such that an electrically
conductive path passes through a junction between the zinc oxide
ceramic layer and the bismuth oxide layer.
This method can be used to relatively easily manufacture a voltage
nonlinear resistive element that includes a plurality of stacked
joined bodies in a voltage nonlinear resistor. Furthermore, because
the heat treatment temperature in the step (b) ranges from
300.degree. C. to 700.degree. C. and is relatively low, this can
prevent the volume resistivity of the zinc oxide ceramic layers
from being increased by heat. In particular, this effect is
significant when the heat treatment temperature in the step (b)
ranges from 300.degree. C. to 500.degree. C.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a cross-sectional view of a voltage nonlinear resistive
element 10.
FIG. 2 is a cross-sectional view of a voltage nonlinear resistive
element 30.
FIG. 3 is a cross-sectional view of a voltage nonlinear resistive
element 130.
FIG. 4 is a cross-sectional view of a voltage nonlinear resistive
element 40.
FIG. 5 is a cross-sectional view of a voltage nonlinear resistive
element 140.
FIG. 6 is a graph of the electric current-voltage characteristics
of voltage nonlinear resistive elements according to Examples 1 and
2 and Comparative Examples 1 to 3.
FIG. 7 is a graph of the relationship between the volume
resistivity (.OMEGA.cm) of a zinc oxide ceramic layer and clamping
voltage at an electric current corresponding to 20 A/cm.sup.2.
DETAILED DESCRIPTION OF THE INVENTION
Preferred embodiments of the present invention will be described
below with reference to the accompanying drawings. FIG. 1 is a
cross-sectional view of a voltage nonlinear resistive element 10
according to the present embodiment.
The voltage nonlinear resistive element 10 includes a voltage
nonlinear resistor (hereinafter abbreviated to a resistor) 14 and a
pair of electrodes 16 and 18 disposed on opposite sides of the
resistor 14.
The resistor 14 includes a joined body 12 in which a zinc oxide
ceramic layer 12a is joined to a bismuth oxide layer 12b composed
mainly of bismuth oxide. The zinc oxide ceramic layer 12a is a
layer composed mainly of zinc oxide and having a volume resistivity
of 1.0.times.10.sup.-1 .OMEGA.cm or less. The volume resistivity is
preferably 1.0.times.10.sup.-3 .OMEGA.cm or less. This can result
in a still lower clamping voltage in a high-current region than
before. The bismuth oxide layer 12b preferably has a thickness in
the range of 0.01 to 1 .mu.m, more preferably 0.05 to 0.5
.mu.m.
The pair of electrodes 16 and 18 are formed on the resistor 14 such
that an electrically conductive path passes through a junction
between the zinc oxide ceramic layer 12a and the bismuth oxide
layer 12b. The electrodes 16 and 18 may be formed of any
electrically conductive material that can form a satisfactory ohmic
contact with the zinc oxide ceramic, for example, gold, silver,
platinum, or aluminum. When a voltage is applied to the element
such that the electrode 18 is an anode, an electric current can
flow easily. On the other hand, when a voltage is applied to the
element such that the electrode 16 is an anode, the element
exhibits voltage nonlinearity.
An example of the manufacture of the voltage nonlinear resistive
element 10 will be described below.
Formation of Zinc Oxide Ceramic Layer 12a
The zinc oxide ceramic layer 12a can be formed by cutting a plate
material having a predetermined size from a zinc oxide ceramic
block having a volume resistivity of 1.0.times.10.sup.-1 .OMEGA.cm
or less, preferably 1.0.times.10.sup.-3 .OMEGA.cm or less. The zinc
oxide ceramic block can be produced by dissolving a trivalent ion
of Al, Ga, In, or the like in a zinc oxide ceramic as a dopant or
by introducing an oxygen defect by firing a zinc oxide powder in a
nonoxidizing atmosphere. A zinc oxide ceramic block in which a
dopant is dissolved can be produced by first mixing a zinc oxide
powder with an Al.sub.2O.sub.3, Ga.sub.2O.sub.3, In.sub.2O.sub.3,
or another trivalent metal oxide powder such that the trivalent
metal oxide constitutes 0.05% to 2.0% by mass and forming the mixed
powder into a compact having a predetermined shape. The compact is
then held in a nonoxidizing atmosphere (for example, a nitrogen or
argon atmosphere) at a temperature in the range of 900.degree. C.
to 1200.degree. C. for several hairs and is then fired at a
temperature in the range of 1300.degree. C. to 1500.degree. C. for
several hours. In this manner, a zinc oxide ceramic block having
low volume resistivity can be relatively easily produced. In order
to achieve the target volume resistivity, the mass percentage of
the trivalent metal oxide powder to be mixed with the zinc oxide
powder or the firing temperature may be adjusted. The zinc oxide
powder used as a raw material preferably has an average particle
size in the range of 0.02 to 5 .mu.m. The trivalent metal oxide
powder preferably has an average particle size in the range of 0.01
to 0.5 .mu.m. The trivalent metal oxide powder is preferably an
Al.sub.2O.sub.3 powder. The Al.sub.2O.sub.3 powder may be formed of
.theta.-alumina, .gamma.-alumina, or boehmite. In order to produce
the zinc oxide ceramic block having low volume resistivity by
firing the zinc oxide powder in a nonoxidizing atmosphere, for
example, the zinc oxide powder is fired in a nonoxidizing
atmosphere (for example, a nitrogen or argon atmosphere) at a
temperature in the range of 1300.degree. C. to 1500.degree. C. for
several hours.
Formation of Bismuth Oxide Layer 12b
The bismuth oxide layer 12b may be formed of bismuth oxide alone or
may contain bismuth oxide as a main component and another oxide
(for example, Sb.sub.2O.sub.3, Cr.sub.2O.sub.3, MnO, CoO, ZnO,
and/or SiO.sub.2) as an accessory component. The bismuth oxide
layer 12b formed of bismuth oxide alone may be formed on the zinc
oxide ceramic layer 12a by sputtering a target of bismuth oxide.
Instead of sputtering, vacuum evaporation or ion plating may also
be used. The bismuth oxide layer 12b may also be formed by applying
a paste containing a bismuth oxide powder to the zinc oxide ceramic
layer 12a, drying the paste, and heat-treating the dried product at
a relatively low temperature (for example, in the range of
200.degree. C. to 700.degree. C., preferably 200.degree. C. to
500.degree. C.). The bismuth oxide layer 12b containing an
accessory component may be formed on the zinc oxide ceramic layer
12a by simultaneously sputtering targets of bismuth oxide and the
accessory component. The bismuth oxide layer 12b may also be formed
by applying a paste containing a bismuth oxide powder and an
accessory component powder to the zinc oxide ceramic layer 12a,
drying the paste, and heat-treating the dried product at a
relatively low temperature. In the case of heat treatment, the
volume resistivity of the zinc oxide ceramic layer 12a is less
likely to be increased by heating because the temperature is
relatively low. However, because sputtering can be performed at a
lower temperature, sputtering can eliminate the possibility of such
an increase in volume resistivity.
Formation of Electrodes 16 and 18
In the present embodiment, the resistor 14 includes one joined body
12 in which the zinc oxide ceramic layer 12a is joined to the
bismuth oxide layer 12b. The electrodes 16 and 18 can be formed on
both sides of the resistor 14 by vapor deposition or sputtering of
an electrode material. The electrode material may be gold, silver,
platinum, or aluminum. Plate-shaped electrodes 16 and 18 may be
joined to the resistor 14 with an electrically conductive joint
member.
When a voltage is applied to the voltage nonlinear resistive
element 10 described above in detail such that the electrode 16 is
an anode, the voltage nonlinear resistive element 10 exhibits
voltage nonlinearity. Thus, the clamping voltage in a high-current
region (for example, in the case that an electric current of 20
A/cm.sup.2 flows) can be lower than before. As a result, for
example, even when a large current flows through the voltage
nonlinear resistive element 10 due to static electricity, a rise in
voltage can be decreased, and the dielectric breakdown of the
element can be prevented.
The present invention should not be limited to the embodiments
described above and can be implemented in various aspects within
the scope of the present invention.
For example, although the voltage nonlinear resistive element 10
includes the electrodes 16 and 18 on both sides of the resistor 14
containing one joined body 12, the resistor 14 may be replaced with
a layered resistor composed of a plurality of joined bodies 12.
Such a layered resistor can be used to control the varistor
voltage. Thus, it is possible to manufacture a voltage nonlinear
resistive element that has a varistor voltage suitable for the
intended application. Examples that include a layered resistor are
illustrated in FIGS. 2 to 5.
A voltage nonlinear resistive element 30 illustrated in FIG. 2
includes a layered resistor 34 and electrodes 16 and 18 disposed on
both sides of the layered resistor 34. The layered resistor 34
includes two joined bodies 12 stacked with a conductor layer 33
interposed therebetween. In the resistor 34, a bismuth oxide layer
12b and the conductor layer 33 are disposed between adjacent zinc
oxide ceramic layers 12a. The resistor 34 can be manufactured as
described below. First, two joined bodies 12 are prepared. A zinc
oxide ceramic layer 12a of one of the joined bodies 12 is brought
into contact with a bismuth oxide layer 12b of the other of the
joined bodies 12 with a brazing filler (for example, Au--Ge alloy
foil, Au--Sn alloy foil, or Au--Si alloy foil), which is a
conductor foil, interposed therebetween. These are pressed into a
single piece. The piece is heated at a predetermined joining
temperature (for example, 300.degree. C. to 700.degree. C.,
preferably 300.degree. C. to 500.degree. C.) in an inert atmosphere
for a predetermined time and is then cooled. This allows the
brazing filler to be melted or softened and then solidified and
form the conductor layer 33, thus constituting the resistor 34. The
voltage nonlinear resistive element 30 has the same advantages as
the element 10. Because the layered resistor 34 includes the two
joined bodies 12, the clamping voltage can be approximately twice
the clamping voltage of the voltage nonlinear resistive element 10
according to Example 1. Furthermore, when the resistance of the
zinc oxide ceramic layers 12a is decreased by adding a large amount
of dopant (trivalent ion) to a zinc oxide ceramic or by forming a
large number of oxygen deficiencies by heat treatment in an inert
atmosphere, an excessively high joining temperature (for example,
900.degree. C. or 1000.degree. C.) sometimes results in high
resistance due to precipitation of the dopant or elimination of the
oxygen deficiencies. In this embodiment, however, because the
joining temperature is 700.degree. C. or less, preferably
500.degree. C. or less, the zinc oxide ceramic layers 12a can have
consistently low resistance.
A voltage nonlinear resistive element 130 illustrated in FIG. 3
includes a layered resistor 134 and electrodes 16 and 18 disposed
on both sides of the layered resistor 134. The layered resistor 134
includes three joined bodies 12 stacked with a conductor layer 33
interposed therebetween. In the resistor 134, a bismuth oxide layer
12b and the conductor layer 33 are disposed between adjacent zinc
oxide ceramic layers 12a. The voltage nonlinear resistive element
130 has the same basic structure and advantages as the voltage
nonlinear resistive element 30 illustrated in FIG. 2 and is
therefore not described in detail. Because the layered resistor 134
in the element 130 includes the three joined bodies 12, the
clamping voltage can be approximately three times the clamping
voltage of the voltage nonlinear resistive element 10 according to
Example 1.
A voltage nonlinear resistive element 40 illustrated in FIG. 4
includes a layered resistor 44 and electrodes 16 and 18 disposed on
both sides of the layered resistor 44. The layered resistor 44
includes two joined bodies 12 stacked with a conductor layer 43
interposed therebetween. In the resistor 44, a bismuth oxide layer
12b, the conductor layer 43, and another bismuth oxide layer 12b
are disposed between adjacent zinc oxide ceramic layers 12a. The
resistor 44 can be manufactured as described below. First, two
joined bodies 12 are prepared. A bismuth oxide layer 12b of one of
the joined bodies 12 is brought into contact with a bismuth oxide
layer 12b of the other of the joined bodies 12 with the brazing
filler interposed therebetween. These are pressed into a single
piece. In the same manner as in the voltage nonlinear resistive
element 30, heat treatment in an inert atmosphere allows the
brazing filler to be melted or softened and then solidified and
form the conductor layer 43, thus constituting the resistor 44.
When a voltage is applied to the voltage nonlinear resistive
element 40 such that the electrode 16 or the electrode 18 is an
anode, the voltage nonlinear resistive element 40 exhibits voltage
nonlinearity. The clamping voltage of the voltage nonlinear
resistive element 40 is almost the same as the clamping voltage of
the voltage nonlinear resistive element 10 according to Example 1.
Furthermore, for the reason described above for the voltage
nonlinear resistive element 30, the zinc oxide ceramic layers 12a
can have consistently low resistance.
A voltage nonlinear resistive element 140 illustrated in FIG. 5
includes a layered resistor 144 and electrodes 16 and 18 disposed
on both sides of the layered resistor 144. The layered resistor 144
includes two joined bodies 12 and one joined body 42 having a
three-layer structure stacked with a conductor layer 43 interposed
therebetween. In the resistor 144, a bismuth oxide layer 12b, the
conductor layer 43, and another bismuth oxide layer 12b are
disposed between adjacent zinc oxide ceramic layers 12a. The
resistor 144 can be manufactured as described below. First, two
joined bodies 12 are prepared. A zinc oxide ceramic layer 12a is
overlaid with a bismuth oxide layer 12b on both sides thereof,
thereby producing the joined body 42 having the three-layer
structure. A bismuth oxide layer 12b of one of the joined bodies 12
is brought into contact with one of the bismuth oxide layers 12b of
the joined body 42 having the three-layer structure with the
brazing filler interposed therebetween. The other of the bismuth
oxide layers 12b of the joined body 42 having the three-layer
structure is brought into contact with a bismuth oxide layer 12b of
the other of the joined bodies 12 with the brazing filler
interposed therebetween. These are pressed into a single piece. In
the same manner as in the voltage nonlinear resistive element 30,
heat treatment in an inert atmosphere allows the brazing filler to
be melted or softened and then solidified and form the conductor
layer 43, thus constituting the resistor 144. When a voltage is
applied to the voltage nonlinear resistive element 140 such that
the electrode 16 or the electrode 18 is an anode, the voltage
nonlinear resistive element 140 exhibits voltage nonlinearity. The
clamping voltage of the voltage nonlinear resistive element 140 is
approximately twice the clamping voltage of the voltage nonlinear
resistive element 10 according to Example 1.
Although a plurality of joined bodies 12 in the embodiments
illustrated in FIGS. 2 to 5 are joined with the conductor layer 33
or 43 interposed therebetween, the joined bodies 12 may be directly
joined without the brazing filler. Also in such a case, the joined
bodies are preferably joined at a relatively low temperature (for
example, 300.degree. C. to 700.degree. C., preferably 300.degree.
C. to 500.degree. C.).
EXAMPLES
Example 1
Zinc oxide (average particle size 1.5 .mu.m) was mixed with 1% by
mass .theta.-alumina (average particle size 0.02 .mu.m). The
mixture was shaped to form a compact. The compact was held in a
N.sub.2 atmosphere at 1100.degree. C. for 5 hours and was then
fired at 1400.degree. C. for 5 hours, thereby producing a zinc
oxide ceramic block. The zinc oxide ceramic block had a volume
resistivity of 9.3.times.10.sup.-4 .OMEGA.cm. A 10 mm.times.10
mm.times.1 mm zinc oxide ceramic sheet was cut from the zinc oxide
ceramic block. After the sheet was polished and washed, a sputtered
bismuth oxide film (thickness 0.3 .mu.m) was formed on the zinc
oxide ceramic sheet by high-frequency plasma sputtering using a
target of bismuth oxide, thereby producing a joined body. RFS-200
manufactured by ULVAC KIKO, Inc. was used for sputtering. The
deposition conditions were as follows: target size: 80 mm in
diameter, RF output: 20 W, gas pressure (Ar): 2.0 Pa, deposition
time: 15 minutes.
The joined body was directly used as a resistor. An Al vapor
deposition electrode was formed on both sides of the resistor.
Thus, a voltage nonlinear resistive element was manufactured (see
FIG. 1). Electric current-voltage characteristics were measured by
applying a voltage between the electrodes of the voltage nonlinear
resistive element. The electrode on the zinc oxide ceramic sheet
was an anode, and the electrode on the sputtered bismuth oxide film
was a cathode. Table 1 and FIG. 6 show the results. The clamping
voltage at an electric current corresponding to 1 mA/cm.sup.2 was
2.7 V, and the clamping voltage at an electric current
corresponding to 20 A/cm.sup.2 was 4.3 V.
TABLE-US-00001 TABLE 1 Examples Comparative Examples 1 2 4 5 1 2 3
Volume Resistivity of Zinc 9.3 .times. 10.sup.-4 1.1 .times.
10.sup.-4 1.0 .times. 10.sup.-1 8.9 .times. 10.sup.-3 3.0 .times.
10.sup.-1 1.5 .times. 10.sup.-1 2.1 .times. 10.sup.-1 Oxide Ceramic
Layer (.OMEGA.cm) V.sub.1 mA/cm2(V)*.sup.1 2.7 2.7 2.6 2.7 2.5 2.6
2.7 V.sub.20 A/cm2(V)*.sup.2 4.3 3.4 6.1 5.2 10.2 8.8 9.0
*.sup.1Clamping voltage at an electric current corresponding to 1
mA/cm.sup.2 *.sup.2Clamping voltage at an electric current
corresponding to 20 A/cm.sup.2
Example 2
A zinc oxide ceramic block was produced in the same manner as in
Example 1 except that zinc oxide was mixed with 2% by mass
.theta.-alumina. The zinc oxide ceramic block had a volume
resistivity of 1.1.times.10.sup.-4 .OMEGA.cm. A 10 mm.times.10
mm.times.1 mm zinc oxide ceramic sheet was cut from the zinc oxide
ceramic block. The sheet is used to produce a joined body in the
same manner as in Example 1. The joined body was directly used as a
resistor. An Al vapor deposition electrode was formed on both sides
of the resistor. Thus, a voltage nonlinear resistive element was
manufactured. Electric current-voltage characteristics were
measured by applying a voltage between the electrodes of the
voltage nonlinear resistive element in the same manner as in
Example 1. Table 1 and FIG. 6 show the results. The clamping
voltage at an electric current corresponding to 1 mA/cm.sup.2 was
2.7 V, and the clamping voltage at an electric current
corresponding to 20 A/cm.sup.2 was 3.4 V.
Comparative Example 1
Aqueous aluminum nitrate was added to zinc oxide. The amount of
aluminum nitrate was 0.001% by mass on an Al.sub.2O.sub.3 basis.
The water content of the product was adjusted to be 50% by mass by
the addition of water. Minute amounts of binder and antifoaming
agent were added to the product. The product was agitated by
ultrasonic waves for 30 minutes and with an impeller blade for 30
minutes. After agitation, the product was granulated with a spray
dryer to produce a granulated powder. The granulated powder was
passed through a sifter having a sieve opening of 200 .mu.m, was
shaped, was degreased, and was fired in the air at 1400.degree. C.
for 5 hours, thereby producing a zinc oxide ceramic block. The zinc
oxide ceramic block had a volume resistivity of 3.0.times.10.sup.-1
.OMEGA.cm. A 10 mm.times.10 mm.times.1 mm zinc oxide ceramic sheet
was cut from the zinc oxide ceramic block. A sputtered bismuth
oxide film was formed on the zinc oxide ceramic sheet in the same
manner as in Example 1, thereby producing a joined body. The joined
body was directly used as a resistor. An Al vapor deposition
electrode was formed on both sides of the resistor. Thus, a voltage
nonlinear resistive element was manufactured. Electric
current-voltage characteristics were measured by applying a voltage
between the electrodes of the voltage nonlinear resistive element
in the same manner as in Example 1. Table 1 and FIG. 6 show the
results. The clamping voltage at an electric current corresponding
to 1 mA/cm.sup.2 was 2.5 V, and the clamping voltage at an electric
current corresponding to 20 A/cm.sup.2 was 10.2 V.
Comparative Example 2
A zinc oxide ceramic block was produced in the same manner as in
Comparative Example 1 except that the aqueous aluminum nitrate was
added to zinc oxide such that the amount of aluminum nitrate was
0.002% by mass on an Al.sub.2O.sub.3 basis. The zinc oxide ceramic
block had a volume resistivity of 1.5.times.10.sup.-1 .OMEGA.cm. A
10 mm.times.10 mm.times.1 mm zinc oxide ceramic sheet was cut from
the zinc oxide ceramic block. The sheet is used to produce a joined
body in the same manner as in Example 1. The joined body was
directly used as a resistor. An Al vapor deposition electrode was
formed on both sides of the resistor. Thus, a voltage nonlinear
resistive element was manufactured. Electric current-voltage
characteristics were measured by applying a voltage between the
electrodes of the voltage nonlinear resistive element in the same
manner as in Example 1. Table 1 and FIG. 6 show the results. The
clamping voltage at an electric current corresponding to 1
mA/cm.sup.2 was 2.6 V, and the clamping voltage at an electric
current corresponding to 20 A/cm.sup.2 was 8.8 V.
Comparative Example 3
Co.sub.3O.sub.4, MnO.sub.2, and NiO powders were added to zinc
oxide such that the amounts of Co.sub.3O.sub.4, MnO.sub.2, and NiO
were 1.2%, 0.50%, and 0.47% by mass of the amount of zinc oxide,
respectively. An aqueous aluminum nitrate was added to the zinc
oxide such that the amount of aluminum nitrate was 0.0018% by mass
on an Al.sub.2O.sub.3 basis. The mixed powder was shaped and was
subjected to hot-press firing in the air at 1300.degree. C. for 1
hour, thereby producing a zinc oxide ceramic block. The zinc oxide
ceramic block had a volume resistivity of 2.1.times.10.sup.-1
.OMEGA.cm. A 10 mm.times.10 mm.times.1 mm zinc oxide ceramic sheet
was cut from the zinc oxide ceramic block. The sheet is used to
produce a joined body in the same manner as in Example 1. The
joined body was directly used as a resistor. An Al vapor deposition
electrode was formed on both sides of the resistor. Thus, a voltage
nonlinear resistive element was manufactured. Electric
current-voltage characteristics were measured by applying a voltage
between the electrodes of the voltage nonlinear resistive element
in the same manner as in Example 1. Table 1 and FIG. 6 show the
results. The clamping voltage at an electric current corresponding
to 1 mA/cm.sup.2 was 2.7 V, and the clamping voltage at an electric
current corresponding to 20 A/cm.sup.2 was 9.0 V.
Voltage generated in a high-current region (for example, a region
of 20 A/cm.sup.2 or more) was lower in the voltage nonlinear
resistive elements according to Examples 1 and 2, which included
the zinc oxide ceramic sheet having a volume resistivity of
1.0.times.10.sup.-1 .OMEGA.cm or less (particularly
1.0.times.10.sup.-3 .OMEGA.cm or less), than in the voltage
nonlinear resistive element according to Comparative Example 1,
which included the zinc oxide ceramic sheet having a volume
resistivity of 3.0.times.10.sup.-1 .OMEGA.cm. Thus, the voltage
nonlinear resistive elements according to Examples 1 and 2 had an
improved electronic circuit protective function.
Example 3
Two joined bodies were prepared in the same manner as in Example 2.
A zinc oxide ceramic sheet of one of the joined bodies was brought
into contact with a sputtered bismuth oxide film of the other of
the joined bodies with a Au--Ge alloy (the mass ratio of
Au/Ge=88/12) foil (thickness 50 .mu.m) interposed therebetween. The
joined bodies in this state were joined by heat treatment in an
inert atmosphere at 420.degree. C. for 10 minutes, thereby
producing a layered resistor. An Al vapor deposition electrode was
formed on both sides of the resistor. Thus, a voltage nonlinear
resistive element was manufactured (see FIG. 2). Electric
current-voltage characteristics were measured by applying a voltage
between the electrodes of the voltage nonlinear resistive element.
The electrode on the zinc oxide ceramic sheet was an anode, and the
electrode on the sputtered film was a cathode. As a result,
although not shown in the figure, the voltage nonlinear resistive
element had the voltage nonlinear resistance characteristics
similar to that in Example 2. The clamping voltage in a
nonlinearity region (a region having an electric current in the
range of 0.01 to 2 A/cm.sup.2) in this case was approximately twice
the clamping voltage in Example 2.
Example 4
A zinc oxide ceramic block was produced in the same manner as in
Comparative Example 1 except that the aqueous aluminum nitrate was
added to zinc oxide such that the amount of aluminum nitrate was
0.1% by mass on an Al.sub.2O.sub.3 basis. The zinc oxide ceramic
block had a volume resistivity of 1.0.times.10.sup.-1 .OMEGA.cm. A
10 mm.times.10 mm.times.1 mm zinc oxide ceramic sheet was cut from
the zinc oxide ceramic block. The sheet is used to produce a joined
body in the same manner as in Example 1. The joined body was
directly used as a resistor. An Al vapor deposition electrode was
formed on both sides of the resistor. Thus, a voltage nonlinear
resistive element was manufactured. Electric current-voltage
characteristics were measured by applying a voltage between the
electrodes of the voltage nonlinear resistive element in the same
manner as in Example 1. Table 1 and FIG. 6 show the results. The
clamping voltage at an electric current corresponding to 1
mA/cm.sup.2 was 2.6 V, and the clamping voltage at an electric
current corresponding to 20 A/cm.sup.2 was 6.1 V.
Example 5
A zinc oxide ceramic block was produced in the same manner as in
Example 1 except that zinc oxide was mixed with 0.5% by mass
.theta.-alumina. The zinc oxide ceramic block had a volume
resistivity of 8.9.times.10.sup.-3 .OMEGA.cm. A 10 mm.times.10
mm.times.1 mm zinc oxide ceramic sheet was cut from the zinc oxide
ceramic block. The sheet is used to produce a joined body in the
same manner as in Example 1. The joined body was directly used as a
resistor. An Al vapor deposition electrode was formed on both sides
of the resistor. Thus, a voltage nonlinear resistive element was
manufactured. Electric current-voltage characteristics were
measured by applying a voltage between the electrodes of the
voltage nonlinear resistive element in the same manner as in
Example 1. Table 1 and FIG. 6 show the results. The clamping
voltage at an electric current corresponding to 1 mA/cm.sup.2 was
2.7 V, and the clamping voltage at an electric current
corresponding to 20 A/cm.sup.2 was 5.2 V.
A graph in FIG. 7 shows the relationship between the volume
resistivity (.OMEGA.cm) of a zinc oxide ceramic layer and clamping
voltage at an electric current corresponding to 20 A/cm.sup.2 on
the basis of Examples 1, 2, 4, and 5 and Comparative Examples 1 to
3. FIG. 7 shows that when the volume resistivity of a zinc oxide
ceramic layer crosses a threshold of 1.0.times.10.sup.-1 .OMEGA.cm,
the clamping voltage at an electric current corresponding to 20
A/cm.sup.2 changes significantly. More specifically, when the
volume resistivity of a zinc oxide ceramic layer is more than
1.0.times.10.sup.-1 .OMEGA.cm, this results in high clamping
voltages at an electric current corresponding to 20 A/cm.sup.2, and
when the volume resistivity is 1.0.times.10.sup.-1 .OMEGA.cm or
less, this results in very low clamping voltages.
Example 6
A 10 mm.times.10 mm.times.1 mm zinc oxide ceramic sheet was cut
from a zinc oxide ceramic block produced in the same manner as in
Example 2. After the sheet was polished and washed, a sputtered
film composed mainly of bismuth oxide (thickness 0.3 .mu.m) was
formed on the zinc oxide ceramic sheet by high-frequency plasma
sputtering using a target, thereby producing a joined body. The
target was produced by mixing Sb.sub.2O.sub.3, CO.sub.3O.sub.4, and
MnO.sub.2 powders with bismuth oxide such that the amounts of
Sb.sub.2O.sub.3, Co.sub.3O.sub.4, and MnO.sub.2 were 2.5%, 6.7%,
and 1.8% by mass of the amount of bismuth oxide, respectively,
shaping the mixture, and firing the shaped mixture in the air at
700.degree. C. for 2 hairs. RFS-200 manufactured by ULVAC KIKO,
Inc. was used for sputtering. The deposition conditions were as
follows: target size: 80 mm in diameter, RF output: 20 W, gas
pressure (Ar): 2.0 Pa, deposition time: 20 minutes. The joined body
was directly used as a resistor. An Al vapor deposition electrode
was formed on both sides of the resistor. Thus, a voltage nonlinear
resistive element was manufactured. Electric current-voltage
characteristics were measured by applying a voltage between the
electrodes of the voltage nonlinear resistive element in the same
manner as in Example 1. As a result, although not shown in the
figure, the voltage nonlinear resistive element had the voltage
nonlinear resistance characteristics similar to that in Example 2.
Table 2 shows the results. The clamping voltage at an electric
current corresponding to 1 mA/cm.sup.2 was 2.6 V, and the clamping
voltage at an electric current corresponding to 20 A/cm.sup.2 was
3.2 V.
TABLE-US-00002 TABLE 2 Comparative Examples Examples 6 7 8 4 Heat
Treatment Temperature (.degree. C.) -- 420 700 900 V.sub.1
mA/cm2(V)*.sup.1 2.6 5.2 5.3 5.3 V.sub.20 A/cm2(V)*.sup.2 3.2 6.4
6.6 15.0 *.sup.1Clamping voltage at an electric current
corresponding to 1 mA/cm.sup.2 *.sup.2Clamping voltage at an
electric current corresponding to 20 A/cm.sup.2
Example 7
Two joined bodies were prepared in the same manner as in Example 6.
A zinc oxide ceramic sheet of one of the joined bodies was brought
into contact with a sputtered bismuth oxide film of the other of
the joined bodies with a Au--Ge alloy (the mass ratio of
Au/Ge=88/12) foil (thickness 50 .mu.m) interposed therebetween. The
joined bodies in this state were joined by heat treatment in an
inert atmosphere at 420.degree. C. for 10 minutes, thereby
producing a layered resistor. An Al vapor deposition electrode was
formed on both sides of the resistor. Thus, a voltage nonlinear
resistive element was manufactured (see FIG. 2). Electric
current-voltage characteristics were measured by applying a voltage
between the electrodes of the voltage nonlinear resistive element.
The electrode on the zinc oxide ceramic sheet was an anode, and the
electrode on the sputtered film was a cathode. Electric
current-voltage characteristics were measured by applying a voltage
between the electrodes of the voltage nonlinear resistive element
in the same manner as in Example 1. As a result, although not shown
in the figure, the voltage nonlinear resistive element had the
voltage nonlinear resistance characteristics similar to that in
Example 2. Table 2 shows the results. The clamping voltage at an
electric current corresponding to 1 mA/cm.sup.2 was 5.2 V, and the
clamping voltage at an electric current corresponding to 20
A/cm.sup.2 was 6.4 V. These clamping voltages are twice the
clamping voltages in Example 6.
Example 8
Two joined bodies were prepared in the same manner as in Example 6.
A zinc oxide ceramic sheet of one of the joined bodies was brought
into contact with a sputtered bismuth oxide film of the other of
the joined bodies with a Ag foil (thickness 20 .mu.m) interposed
therebetween. The joined bodies were joined by heat treatment in an
inert atmosphere at 700.degree. C. for 30 minutes, thereby
producing a layered resistor. An Al vapor deposition electrode was
formed on both sides of the resistor. Thus, a voltage nonlinear
resistive element was manufactured (see FIG. 2). Electric
current-voltage characteristics were measured by applying a voltage
between the electrodes of the voltage nonlinear resistive element.
The electrode on the zinc oxide ceramic sheet was an anode, and the
electrode on the sputtered film was a cathode. Electric
current-voltage characteristics were measured by applying a voltage
between the electrodes of the voltage nonlinear resistive element
in the same manner as in Example 1. As a result, although not shown
in the figure, the voltage nonlinear resistive element had the
voltage nonlinear resistance characteristics similar to that in
Example 2. Table 2 shows the results. The clamping voltage at an
electric current corresponding to 1 mA/cm.sup.2 was 5.3 V, and the
clamping voltage at an electric current corresponding to 20
A/cm.sup.2 was 6.6 V. These clamping voltages are approximately
twice the clamping voltages in Example 6.
Comparative Example 4
Two joined bodies were prepared in the same manner as in Example 6.
A zinc oxide ceramic sheet of one of the joined bodies was brought
into contact with a sputtered bismuth oxide film of the other of
the joined bodies with a Ag foil (thickness 20 .mu.m) interposed
therebetween. The joined bodies were joined by heat treatment in an
inert atmosphere at 900.degree. C. for 30 minutes, thereby
producing a layered resistor. An Al vapor deposition electrode was
formed on both sides of the resistor. Thus, a voltage nonlinear
resistive element was manufactured (see FIG. 2). Electric
current-voltage characteristics were measured by applying a voltage
between the electrodes of the voltage nonlinear resistive element.
The electrode on the zinc oxide ceramic sheet was an anode, and the
electrode on the sputtered film was a cathode. Electric
current-voltage characteristics were measured by applying a voltage
between the electrodes of the voltage nonlinear resistive element
in the same manner as in Example 1. As a result, depending on
resistors thus manufactured, some resistive elements exhibited
voltage nonlinear resistance characteristics, and some resistive
elements did not exhibit voltage nonlinear resistance
characteristics. Table 2 shows the results. For the resistive
elements that exhibited voltage nonlinear resistance
characteristics, the clamping voltage at an electric current
corresponding to 1 mA/cm was 5.3 V, and the clamping voltage at an
electric current corresponding to 20 A/cm.sup.2 was 15.0 V, which
is still more than twice the clamping voltage in Example 6.
Examples 6 to 8 and Comparative Example 4 show that when a bismuth
oxide layer contains an oxide other than bismuth oxide
(Sb.sub.2O.sub.3, Co.sub.3O.sub.4, and MnO.sub.2 in these
examples), joining of joined bodies by heat treatment at a
temperature of more than 700.degree. C. results in unstable voltage
nonlinear characteristics and high clamping voltages. Thus, the
resulting voltage nonlinear resistive elements may have high
voltages in a high-current region.
These examples are only examples of the present invention and do
not limit the present invention.
This application claims priorities from Japanese patent application
No. 2014-056497 filed on Mar. 19, 2014, the entire contents of both
of which are incorporated herein by reference.
* * * * *